Magnetic tapes have found various applications in audio tapes, videotapes, computer tapes, etc. In particular, in the field of magnetic tapes for data-backup, tapes with recording capacities of several tens to 100 GB per reel are commercialized in association with increased capacities of hard discs for back-up. Under such circumstances, a backup tape with a capacity of as large as 100 GB or more has been proposed, and it is indispensable for such a backup tape to have a higher recording density.
In the production of a magnetic tape capable of meeting such a demand for higher recording density, the use of very fine ferromagnetic powder improved in magnetic properties is needed, and further improvements of the filling property and dispersibility of the ferromagnetic powder are needed. As the wavelengths of signals for use in recording becomes shorter and shorter, it is more and more necessary to reduce the thickness of a magnetic layer so as to lessen demagnetization due to a demagnetizing field which is caused during the recording/reproducing of data.
To improve the magnetic properties of ferromagnetic powder, it is desirable that residual magnetization of a magnetic layer should be large so as to obtain higher output. For this purpose, the use of ferromagnetic iron-based metal powder as magnetic powder has been prevailed over the use of conventional oxide magnetic powder and cobalt-containing iron oxide magnetic powder. For example, ferromagnetic iron-based metal powder having a coercive force (Hc) of 119 kA/m (1,500 Oe) or more is proposed (JP-A-5-234064, JP-A-6-25702, JP-A-6-139553, etc.). Further, there are proposed rare earth-iron-based magnetic powder, such as rare earth-iron-boron-based magnetic powder (JP-A-2001-181754) which comprises substantially spherical or ellipsoidal particles in which the ratio of the major axis/the minor axis is 1 to 2, and rare earth-iron-based magnetic powder (JP-A-2002-56518). As the rare earth elements for use in any of these magnetic powders, at least one element selected from the group consisting of yttrium, ytterbium, cesium, praseodymium, samarium, lanthanum, europium, neodymium and terbium is used. Among them, neodymium (Nd), samarium (Sm), terbium (Tb) and yttrium (Y) are preferably used. Other than those, there is known iron nitride-based magnetic powder which comprises substantially spherical (or cubic) particles having a phase of Fe16N2 as a main phase and a BET specific surface area of 10 m2/g or more (JP-A-2000-277311). The coercive forces of these magnetic powders which comprise substantially spherical or ellipsoidal particles are 80 kA/m or more.
To improve the magnetic properties of a magnetic layer comprising ferromagnetic powder, it is effective to improve the dispersibility of the ferromagnetic powder. To improve the dispersibility of the ferromagnetic powder, the following methods are proposed: a binder having a polar functional group such as a sulfonic acid group, phosphoric acid group or a salt of any of these acids with an alkali metal is used; a dispersant having a low molecular weight is used in combination with a binder; the step of kneading and dispersing a magnetic coating composition is continuously carried out; a lubricant is added after the dispersion; and so on (cf. JP-A-2-101624, JP-A-3-216812, JP-A-3-17827, JP-A-8-235566, etc.).
To improve the magnetic properties of the magnetic layer, it is also necessary for the magnetic layer to contain magnetic powder at high density. Generally, the amount of the magnetic powder contained in the magnetic layer per unit area is expressed by a Mrt value (i.e., a product of the residual magnetic flux density and the thickness of the magnetic layer, which is hereinafter simply referred to as “Mrt”), and as this value increases, the resultant reproducing output increases. However, when the value of Mrt is increased, the value of PW50 of a solitary waveform concurrently increases. The value of PW50 herein referred to is a value which expresses a peak half-value width of a reproduced solitary waveform, namely, the reproduced waveform of a signal recorded on a magnetic recording medium by one pulse of recording current, in unit of length. This value, hereinafter, is simply referred to as “PW50”. Since a larger value of PW50 indicates a decrease in resolving power, it is impossible to record or reproduce signals at a high recording density. For this reason, it is meaningless to unnecessarily increase the value of Mrt.
On the other hand, the latest development of reproduction systems utilizing MR heads (reproduction magnetic heads comprising magnetoresistance effect elements) further accelerates the shortening of recording wavelengths. For example, a recent model of digital data storage system can record signals with the shortest wavelengths of 500 nm or less. When MR heads are used, saturation of MR elements (magnetoresistance effect elements) occurs when a Mrt value to each of the MR elements exceeds a suitable value. As a result, the increment of noises as a background increases, although an output is hardly increased. Consequently, the value of C/N, detected when signals are actually reproduced, becomes smaller, and efficient electromagnetic conversion cannot be attained (cf. Japanese Patent No. 3046579 and JP-A-10-134306). Therefore, it is necessary to decrease the thickness of a magnetic layer to one third or less of the shortest recording wavelength, on the assumption that the residual magnetic flux density is not changed. This means that a very thin magnetic layer with a thickness of 100 nm or less is needed for a magnetic recording/reproducing system, as mentioned above.
In the meantime, other than the high performance magnetic layer as mentioned above, a magnetic recording medium suitable for recording signals with short wavelengths is proposed in which a lower non-magnetic layer is provided between a non-magnetic support and a thin magnetic layer with a thickness of 600 nm or less (cf. JP-A-5-234063). In this magnetic recording medium, the upper magnetic layer is formed with a reduced thickness to optimize the value of Mrt and reduce self-demagnetization loss and reproduction loss, and also to suppress the deterioration of traveling performance and durability of the magnetic medium which are induced by the decrease of the thickness of a magnetic layer.
Further, in the system using the MR head, when the number of ferromagnetic particles in the volume of reversal of magnetization is larger, noises can be decreased to achieve a higher ratio of C/N. This is advantageous to achieve high recording density, and for such high density recording, it is necessary to use micronized magnetic particles with major axes of 100 nm or less.
However, there is a limit in the formation of a thinner magnetic layer by providing a lower non-magnetic layer as mentioned above. As a matter of fact, it is difficult to form a magnetic layer with a thickness of 100 nm or less, in view of the controlling of a layer thickness and the productivity. Even if a magnetic layer with a thickness of approximate 100 nm could be formed, it would be very difficult to control, to 50 nm or less, the interfacial fluctuation at the interface between the upper magnetic layer and the lower non-magnetic layer. When the v fluctuation at the interface between the upper and lower layers is large, modulation noise (a noise with a frequency close a carrier frequency) becomes higher, so that it is impossible to achieve efficient electromagnetic conversion.
JP-A-5-290353 proposes the provision of an intermediate layer between an upper magnetic layer and a lower non-magnetic layer. However, this invention intends to orient the particles in the upper magnetic layer obliquely or vertically, and the thickness of the intermediate layer is 100 to 800 nm. Thus, the fluctuation at the interface between the upper magnetic layer and the intermediate layer, on the contrary, becomes very large. Further, because of the thick intermediate layer, a lubricant cannot be fed from the lower non-magnetic layer to the upper magnetic layer. As a result, the durability of the magnetic tape becomes poor.